1. Field of the Invention
The present invention relates to a new and improved light emitting diode that is encapsulated with optically absorbing material to reduce stray light, and to a new and improved method for encapsulating a light emitting diode to reduce stray light.
2. Description of the Background Art
In designing blood glucose monitoring instruments, the direction of development has been toward smaller instruments that are lower in cost. To meet these goals, more than one function is designed into a single component. This approach lowers the part count, reduces the size of the instrument and increases the reliability.
One example of this trend is the use of a light emitting diode (LED) to replace the combination of an incandescent lamp, and an interference filter in the optical system of blood glucose monitoring instruments. Examples of instruments that use an LED as a light source include the GLUCOMETER III manufactured by Miles, Inc., the ACCUCHECK manufactured by Boehringer Mannheim GmbH and the GLUCOSCAN manufactured by Lifescan, Inc. Examples of devices using an LED in a readhead are to be found in U.S. Pat. Nos. 4,552,458; and 4,676,653 and European Patent Application No. 83304306.6, filed Jul. 26, 1983. LED light sources have also been used in other diagnostic instruments such as the instrument disclosed in U.S. Pat. No. 4,755,058. The typical LED used as a light source in the readhead of a blood glucose monitoring system is the standard T-13/4 which can be purchased from manufacturers such as the ES Series made by Stanley Electric Co., Ltd. or the HLMP Series from Hewlett-Packard.
One of the problems in using a standard LED as a light source in a blood glucose monitoring instrument is the stray light emitted by the LED. In a diagnostic instrument readhead it is preferable that the LED emission be generally collimated toward a small sample area. Rays emitted at wide angles are undesirable stray light that preferably are minimized or eliminated, since stray light in an optical system of a readhead instrument manifests itself as a background signal.
At least three procedures have been used in instruments to control the stray light from an LED. One method is to ignore the existence of stray light. By ignoring the stray light, however, there can be a high level of instrument to instrument imprecision. Due to this imprecision, ignoring the stray light has not been considered an acceptable method.
Another method of controlling stray light is to accept only a small solid angle of the emission from an LED. Such an approach is used in the instrument described in U.S. Pat. No. 4,676,653. This approach results in an instrument that uses only a small portion of the available light from the LED. This approach also reduces the magnitude of the available signal and can make the optical system more susceptible to noise.
Another approach is to use a shield to avoid the detection of the stray light. Such an approach is illustrated in European Patent Application No. 83304306.6. This approach does not shield the stray light flaring in directions other than in direct line with the shield. Commuter simulations have been performed that suggest that there is a significant amount of this light that flares in all directions after emission from an LED, and for this reason, shielding is not preferred.